With the advent of modern antipollution laws in the United States and around the world, significant and new methods of minimizing various pollutants are being investigated. The burning of fuel--be the fuel wood, coal, oil, or a natural gas--likely causes a majority of the pollution problems in existence today. Certain pollutants, such as SO.sub.2, which are created as the result of the presence of a contaminant in the fuel source may be removed either by treating the fuel to remove the contaminant or by treating the exhaust gas eventually produced. Other pollutants such as carbon monoxide, which are created as the result of imperfect combustion, may be removed by post-combustion oxidation or by improving the combustion process. The other principal pollutant, NO.sub.x (an equilibrium mixture mostly of NO, but also containing very minor amounts of NO.sub.2), may be dealt with either by controlling the combustion process to minimize NO.sub.x production or by later removal. Removal of NO.sub.x, once produced, is a difficult task because of its relative stability and its low concentrations in most exhaust gases. One ingenious solution used in automobiles is the use of carbon monoxide to reduce NO.sub.x to nitrogen while oxidizing the carbon monoxide to carbon dioxide. However, the need to react two pollutants also speaks to a conclusion that the initial combustion reaction was inefficient.
It must be observed that unlike the situation with sulfur pollutants where the sulfur contaminant may be removed from the fuel, removal of nitrogen from the air fed to the combustion process is an impractical solution. Unlike the situation with carbon monoxide, improvement of the combustion reaction would likely increase the level of NO.sub.x produced due to the higher temperatures then involved.
Nevertheless, the challenge to reduce NO.sub.x remains and several different methods have been suggested. The NO.sub.x abatement process chosen must not substantially conflict with the goal for which the combustion gas was created, i.e., the recovery of its heat value in a turbine, boiler, or furnace.
Many recognize that a fruitful way to control NO.sub.x production is to limit the localized and bulk temperatures in the combustion zone to something less than 1800.degree. C. See, for instance, U.S. Pat. No. 4,731,989 to Furuya et al. at column 1, lines 52-59 and U.S. Pat. No. 4,088,435 to Hindin et al. at column 12.
There are a number of ways of controlling the temperature, such as by dilution with excess air, by controlled oxidation using one or more catalysts, or by staged combustion using variously lean or rich fuel mixtures. Combinations of these methods are also known.
One widely attempted method is the use of multistage catalytic combustion. Most of these processes utilize multi-section catalysts with metal or metal oxide catalysts on ceramic catalyst carriers. Typical of such disclosures are:
__________________________________________________________________________ Country Document 1st Stage 2nd Stage 3rd Stage __________________________________________________________________________ Japan Kokai 60-205129 Pt-group/Al.sub.2 O.sub.3 &SiO.sub.2 La/SiO.sub.2.Al.sub.2 O.sub.3 Japan Kokai 60-147243 La&Pd&Pt/Al.sub.2 O.sub.3 ferrite/Al.sub.2 O.sub.3 Japan Kokai 60-66022 Pd&Pt/ZrO.sub.2 Ni/ZrO.sub.2 Japan Kokai 60-60424 Pd/- CaO&Al.sub.2 O.sub.3 &NiO&w/noble metal Japan Kokai 60-51545 Pd/* Pt/* LaCoO.sub.3 /* Japan Kokai 60-51543 Pd/* Pt/* Japan Kokai 60-51544 Pd/* Pt/* base metal oxide/* Japan Kokai 60-54736 Pd/* Pt or Pt--Rh or Ni base metal oxide or LaCO.sub.3 /* Japan Kokai 60-202235 MoO.sub.4 /- CoO.sub.3 &ZrO.sub.2 &noble metal Japan Kokai 60-200021 Pd&Al.sub.2 O.sub.3 /+* Pd&Al.sub.2 O.sub.3 /** Pt/** Japan Kokai 60-147243 noble metal/heat ferrite/heat resistant carrier resistant carrier Japan Kokai 60-60424 La or Nd/Al.sub.2 O.sub.3 0.5% SiO.sub.2 Pd or Pt/NiO&Al.sub.2 O.sub.3 & CaO 0.5% SiO Japan Kokai 60-14938 Pd/? Pt/? Japan Kokai 60-14939 Pd&Pt/refractory ? ? Japan Kokai 61-252409 Pd&Pt/*** Pd&Ni/*** Pd&Pt/*** Japan Kokai 62-080419 Pd&Pt Pd,Pt&NiO Pt or Pt&Pd Japan Kokai 62-080420 Pd&Pt&NiO Pt Pt&Pd Japan Kokai 63-080848 Pt&Pd Pd&Pt&NiO Pt or Pt&Pd Japan Kokai 63-080849 Pd,Pt,NiO/? Pd&Pt(orNiO)/? Pt or Pd&Pt/? __________________________________________________________________________ *alumina or zirconia on mullite or cordierite **Ce in first layer; one or more of Zr, Sr, Ba in second layer; at least one of La and Nd in third layer. ***monolithic support stabilized with lanthanide or alkaline earth metal oxide Note: the catalysts in this Table are characterized as "a"/"b" where "a" is the active metal and "b" is the carrier
It is, however, difficult to control the temperatures in these processes. Since the object of each of the processes is to produce a maximum amount of heat in a form which can be efficiently used in some later process, the combustion steps are essentially adiabatic. Consequently, a minor change in any of fuel rate, air rate, or operating processes in an early stage will cause significant changes in the latter stage temperatures. Very high temperatures place thermal strain on downstream catalytic elements.
This list also makes clear that platinum group metals (including palladium) are considered useful in catalytic combustion processes. However, conventional catalytic combustion processes often mix the fuel and air and then pass this mixture over a catalyst with essentially complete combustion in the catalyst bed. This results in extremely high temperatures, typically 1100.degree. C. to 1500.degree. C. For this reason, much of the catalyst development work is directed at catalysts and supports that can withstand those high temperatures and yet remain active. Some have relied on process control schemes in which the flow rate of an intermediate stream of air or fuel is introduced between catalyst stages and is controlled based upon bulk gas temperature. Furuya et al. (mentioned above) describes one approach in circumventing the problems associated with a high catalyst temperature through dilution of the fuel/air mixture with air fed to the catalyst so that the resulting mixture has an adiabatic combustion temperature of 900.degree. C. to 1000.degree. C. This mixture is passed through the catalyst and partial or complete reaction gives a maximum catalyst temperature less than 1000.degree. C. and a gas temperature less than 1000.degree. C. Additional fuel is added after the catalyst and homogeneous combustion of this mixture gives the required temperature (1200.degree. C. to 1500.degree. C.). This process, however, suffers from the need to add fuel at two stages and the requirements to mix this additional fuel with hot gases without obtaining a conventional high temperature diffusion flame and the associated production of NO.sub.x.
We have found that a carefully constructed catalyst structure involving self-exchange of combustion heat into the flowing gas is an excellent way to control the temperature of the catalyst and the catalyst support. The metallic supports as we use them are much more flexible in their ability to control heal loads in a reaction and to provide long term chemical and physical stability. Use of such supports eliminates the need for the complicated air dilution procedures such as shown in Furuya et al.
However, the use of metal catalyst supports for platinum group metals has been suggested in passing. See, for instance, U.S. Pat. No. 4,088,435 (to Hindin et al.), "platinum group metals" at column 4, lines 63 et seg., and "the support may be metallic or ceramic. . . " at column 6, line 45. Conversely, the use of a platinum group alloy catalyst on a monolithic support as a combustion catalyst is suggested in U.S. Pat. No. 4,287,856 (to Enga) at column 1, line 65 et al. Other similar disclosures are found in the earlier U.S. Pat. , Nos. 3,966,391; 3,956,188; and 4,008,037 (all to Hindin et al.). Platinum on a steel ("Fecralloy") support as a combustion catalyst for low heating value gas is suggested in U.S. Pat. No. 4,366,668 (to Madgavkar et al.). Other disclosures of metals and metal supports used mainly for automotive catalytic converters include:
______________________________________ Country Document Assignee ______________________________________ U.S. 3,920,583 Pugh U.S. 3,969,082 Cairns et al. U.S. 4,279,782 Chapman et al. U.S. 4,318,828 Chapman et al. U.S. 4,331,631 Chapman et al. U.S. 4,414,023 Aggen et al. U.S. 4,521,532 Cho U.S. 4,601,999 Retallick et al. U.S. 4,673,663 Magnier U.S. 4,742,038 Matsumoto U.S. 4,752,599 Nakamura et al. U.S. 4,784,984 Yamanaka et al. Great Britain 1,528,455 Cairns et al. ______________________________________
As a group, these patents generally discuss ferritic catalyst supports upon which alumina is found as micro-crystals, coatings, whiskers, etc. Many disclose that platinum group metals are suitably placed on those supports as catalysts. None suggest the use of an integral heat exchanger in the catalyst support to aid in the catalyst operation.
Two published Japanese Kokai teach the use of catalyst supports having integral heat exchange surfaces. They are Japanese Kokai 59-136,140 (published Aug. 4, 1984) and Kokai 61-259,013 (published Nov. 17, 1986). The earlier Kokai deals generally with a combustion process using a square-sectioned ceramic monolithic catalytic support in which alternating longitudinal channels (or alternating layers) are shown to have catalysts deposited therein. The monoliths disclosed there are ceramic and are consequently molded, not assembled.
Similarly, the later Kokai discloses a process for the combustion of a fuel gas, in this case a blast furnace off-gas, using a catalyst structure such as that shown in the earlier Kokai or a structure of concentric cylinders in which alternating annular spaces in the support are coated with catalyst. Neither Kokai suggest the use of metal monoliths of any configuration in the processes.
A disclosure very similar to the two Kokai is seen in U.S. Pat. No. 4,870,824 to Young et al.
In summary, although the literature suggests various unrelated portions of the inventive catalyst structure, none of these documents suggests metallic supports produced from a corrugated metal having catalytic material, particularly platinum group metals, deposited only on one side of the corrugated metal which are physically tough and have long term stability.